Viral pathogenesis is an important problem. We plan genetic, biochemical, and biological studies using Newcastle disease virus (NDV) as a model system to analyze paramyxovirus pathogenesis. Our tools include the noncytopathic (nc) mutants of Mandansky and Bratt, temperature sensitive mutants, and revertants of these mutants; all of these are derived from the virulent and cytopathic Australia-Victoria strain (AV-WT) which serves as our principal will type virus. In addition, we describe a variety of different types of mutants and their revertants derived from a selected panel of naturally occurring NDV strains differing in virulence patterns and numerous related properties definable in the laboratory. Where appropriate, we will also employ our expanding library of monoclonal antibodies to NDV proteins. We will continue to explore phenotypes relevant to cytopathogenicity and virulence, hoping to determine the general validity of our previous finding with the nc mutants of AV-WT which have implicated both mRNA modulated cytopathogenicity and cell to cell spread. We will select nc type mutants from another virulent and cytopathic NDV strain which already possesses some characteristics of avirulent strains. Cell to cell spread mutants of this strain have been isolated and will be characterized. A second approach will involve isolating mutants from avirulent strains: these will include plaque forming mutants, screened for in the presence or absence of exogenous protease, and hemadsorption positive spot forming mutants, screened for as previously done for the nc mutants. We will further explore a variety of specific mechanisms which have been suggested to be involved in cytopathogenicity or cell killing. Possible roles of specific proteins will be explored through the use of well characterized ts mutants and neutralization resistant mutants known to be altered in specific regions of the membrane glycoproteins. A major focus will be the exploration of possible roles of these glycoproteins in cell killing. We will also explore the role of the P protein in cell to cell spread. Finally, we will test an hypothesis which invokes a sequestering of ribosomes by non-translatable mRNA in NDV's inhibition of total protein synthesis.